(1) Field of the Invention
The invention relates to a method for establishing (=determining, finding, “measuring”) reflection properties of a given surface, such as a road surface, among others in view of deducing there from the desired illuminance and luminance performances of lighting systems/installations.
The invention more particularly aims at deducing a series of representative parameters of an unknown surface, such as those collectively referred to as “r-table” or “table-r”, specifically representative of the light reflection or reflectance of a road surface, from the measurement, in situ, of converted luminance parameters of said road surface/road covering. at defined angles of the light incidence and of the light reflection.
(2) Description of Related Art
Determination of the reflection properties of a road surface is an important aspect for calculating the characteristics and the luminance level of road lighting installations. Particular reference is made in this respect to the publication “Calculation and Measurement of Luminance and Illuminance in road lighting” from the Commission Internationale de l'Eclairage—CIE (Publication CIE No. 30-2 (TC-4.6) 1982) the entire content of which is incorporated by reference in the present text. In order to design a lighting installation and optimise it in respect of luminance, lighting engineers must be able to predict the luminance levels at the road surface. Several parameters have to be taken into account for that purpose: the intensity distribution of the emitted light which in general is quite well known and defined by the manufacturers of lighting apparatuses; the light flux of the lamps, the geometry of the configuration (width of the road, height of the installation, etc.) and the properties of the road surface.
Luminance calculations are often performed by means of software and more in general by using theoretical road surface characteristics, such as the r-table characteristics defined by the CIE.
Calculations for lighting installations are often based on the characteristics of a very limited number of reference surfaces (such as standard classes R1, R2, R3 and R4 as defined by CIE).
The use of such a limited number of reference surfaces for characterising all imaginable road surfaces clearly suggest that improvements to the method are desirable.
The conceiver of the present invention was one of the pioneers in this area and carried out numerous measurements of road surface properties using a rather sophisticated gonio-reflectometer. This instrument is capable of measuring the behaviour of a road surface both at an observation angle of 1° (α=1°, i.e. the specific viewing angle of a car driver) and at other observation angles (α up to 90°), more appropriate for measuring the characteristics of, for instance, tunnel walls. For carrying out such measurements, it is necessary to extract samples (having a cross section of 100 mm2 to 200 mm2) from the road surface, in order to subsequently measure those in the laboratory where the gonio-reflectometer is installed.
The thus obtained “r-table” characteristics, representing the behaviour of the road surface at a given observation angle (1° for road lighting applications) can then be introduced in appropriate software programmes (well known per se to those skilled in the art) so that luminance of the lighting installations can be predicted with good accuracy. The extraction of road samples is traffic disturbing, time consuming and costly so that in general no more than 2-3 samples are taken and analysed.
The essential question remains however whether a few (2-3) samples may really be representative of the entire surface of a road, knowing that such a road is never quite homogeneous. The answer is clearly “no”.
Therefore the calculated luminance will not really be representative of the actual situation.
A solution to improve this situation would involve taking and analysing more samples, and to perform calculations on the basis of average values, but this has proven too expensive.